1. Field of the Invention
The present invention relates to a drive power transmission device such as an electromagnetic type starting clutch incorporated in a drive power train of a vehicle or such as, in particular, an electromagnetic type coupling interposed between a front wheel drive train and a rear wheel drive train in a four-wheel vehicle.
2. Discussion of the Related Art
A drive power transmission device 1 in the prior art will be described hereafter with reference to FIGS. 13 to 19.
In the prior art device, when an electric current is applied to an electromagnet 33 of an electromagnetic type clutch mechanism 30, a magnetic path (M) schematically shown by the broken line is generated around the electromagnet 33, and an armature 31 is attracted toward the electromagnet 33, so that outer plates 44 and inner plates 45 of a pilot friction clutch 32 are brought into friction contact. When a relative rotation occurs between an outer case 14 and an inner case 17, a torque generated at a cam mechanism 29 is converted into a thrust force acting in the direction of the rotational center axis 14a. A main friction clutch 28 is connected in proportion to the thrust force, whereby the drive power is transmitted between the outer case 14 and the inner case 17.
On the contrary, when no electric current is applied to the electromagnet 33 of the electromagnetic type clutch mechanism 30, the aforementioned magnetic path (M) is not generated around the electromagnet 33, and the armature 31 is released from the attraction toward the electromagnet 33. Thus, the main friction clutch 28 is brought into disconnection thereby to release the driving connection between the outer case 14 and the inner case 17.
The aforementioned magnetic path (M) includes a clutch magnetic path 61 whose magnetic flux passes to reciprocate once across the pilot friction clutch 32. Gas soft-nitriding surface treatment using a special gas is effected on friction contact surfaces of the outer plates 44 and friction contact surfaces of the inner plates 45.
For a smaller size and a reduced cost of the drive power transmission device 1, it may be conceivable to reduce the number of the plates 44, 45 of the pilot friction clutch 32. However, if an attempt is made to reduce the number of the plates 44, 45, the friction contact area which a smaller number of the plates 44, 45 provide is decreased, whereby the drive torque transmittable by the pilot friction clutch 32 may be lowered.